1. Technical Field
Embodiments of the present disclosure are related in general to the management of aircraft engine noise, and in particular, to the management of noise emanating from the inlet of a turbo-fan engine.
2. Description of the Related Art
Jet airplanes are configured such that the propulsion systems, called nacelles, are attached to the airframe either on or near the wings or near the rear fuselage. Among the components of each nacelle is a turbo-fan engine. The front interface of the turbo-fan engine is in contact with an air intake system, called the inlet.
A conventional inlet consists of three segments of surfaces, namely, a contoured interior surface, an inlet lip surface, and a contoured exterior surface. The inlet lip surface and the exterior surface are integral parts of the front portion of the nacelle cowling.
In the 1970's, a flight hardware acoustic nacelle demonstration program was designed, and tests carried out to evaluate what could be done about reducing engine noise. In that program, noise absorption lining materials were installed onto the interior surfaces of the inlet and fan exhaust. It was found that fan exhaust noise could be managed.
However, it was also found that inlet noise was not greatly affected by inlet peripheral linings (inlet acoustic rings were not desirable). The situation was aggravated by the restriction on the flight inlet length—simply put, there was not enough surface area for the needed linings, as compared to that available in the fan exhaust duct.
In addition, buzz saw noise generated in the fan inlet that was a type of noise not generated at the fan exhaust duct. Identification of buzz saw noise signatures and methods of their attenuation were pursued in the 1970's and are continuing up to the present. Buzz saw noise is a dominant inlet noise component at take-off engine speeds. During take-off and climb out operations, the engine fan is operating at a near maximum speed to satisfy the thrust demands. A large portion of the fan blades is operating at supersonic speeds. An uneven circumferentially locked fan rotor shock system generates buzz saw tones at multiple rotor rotational frequencies. To reduce these buzz saw tones, acoustic linings were observed as being more effective, compared to their effectiveness with other inlet noise signatures. However, the effectiveness of acoustic linings at attenuating buzz saw noise was still limited.
It was also found that boundary layer flow development in the inlet degrades the noise reduction performance of inlet acoustic lining.
It would seem logical to apply suction to the Inlet boundary layer flow to remove the boundary layer in order to restore noise reduction performance of the inlet lining to that at laboratory test condition. But from the point of view of inlet noise abatement as a whole, the effect of boundary layer flow suction is limited. The benefit vs. cost balance does not favor the application of boundary layer flow suction. The struggle in dealing with inlet noise and its reduction is an on-going task up to the present.